In order to construct a rotating support member for a variety of machinery, a back-to-back double-row angular ball bearing 1 such as shown in FIG. 7 is widely used. This double-row angular ball bearing 1 comprises; an outer ring 3 having double-row outer ring raceways 2 on the inner peripheral surface, a pair of inner rings 5 formed with inner ring raceways 4 on the respective outer peripheral surfaces, a plurality of balls 6 provided in a freely rolling manner between the outer ring raceways 2 and the inner ring raceways 4 of both inner rings 5, and a pair of cages 7 for holding the balls 6. In such a double-row angular ball bearing 1, for example, the outer ring 3 is fitted internally into a housing 8, and the inner rings 5 are fitted externally to a rotating shaft 9. The rotating shaft 9 is supported inside the housing 8 in a freely rotating manner.
The outer ring 3 and the two inner rings 5 which constitute such a double-row angular ball bearing 1 are processed to a predetermined shape and size by, for example, performing a forging process, a rolling process, and machining and grinding processes in the well-known manner described in patent documents 1 through 5. For example, the outer ring 3 is conventionally manufactured by the processes shown in FIG. 8. First, this conventional manufacturing method for a bearing outer ring is described.
In the known method of manufacturing a bearing outer ring shown in FIG. 8, first a cylindrical raw material 10 as shown by (A) is obtained by cutting a long piece of a raw material into predetermined lengths.
Next, the raw material 10 is subjected to an upsetting process by compressing the raw material 10 in the axial direction between opposing pressing surfaces of a pair of dies, to obtain a first intermediate material 11 whose outer peripheral surface is a convex circular arc as shown by (B).
Next, this first intermediate material 11 is subjected to a backward extrusion process shown by (C) to (D) to obtain a second intermediate material 12 shown by (D).
The backward extrusion process is performed by compressing the radial center portion of the first intermediate material 11 in the axial direction, between a die 13 and a punch 14, and plastically deforming the radial outlying portion in a direction opposite to the pushing direction of the punch 14. The die 13 is a bottomed cylinder, comprising a circular base plate 15 and a peripheral wall portion 16 which extends upward from the outside edge of the base plate 15. An annular groove 17 is formed around the entire periphery of the outlying portion of the base plate 15. Furthermore, the inner peripheral surface of the peripheral wall portion 16 adopts a stepped shape in which an inner periphery large diameter portion 18 on the opening side (from the center portion to the top end) is connected to an inner periphery small diameter portion 19 on the base plate 15 side (bottom end) by an inner periphery inclined portion 20 on the base plate side of the axial center portion. The inner periphery small diameter portion 19 is positioned upon the same cylindrical surface as the outlying inner peripheral surface of the annular groove 17. Furthermore, the outer peripheral surface of the punch 14 adopts a stepped shape in which an outer periphery small diameter portion 21 at the forward end (lower half) is connected to an outer periphery large diameter portion 22 at the base end (top half) by an outer periphery inclined portion 23 at the axial center portion. The die 13 and the punch 14 having these respective constructions are concentrically secured to and supported by a table and ram of a pressing machine. In other words, the die 13 is secured to the top surface of the table and the punch 14 is secured to the bottom end face of the ram.
When performing this backward extrusion process, with the punch 14 and the ram in an elevated state, the first intermediate material 11 is set inside the die 13. In the case of the conventional manufacturing method, the outside diameter of the first intermediate material 11 is smaller than the inside diameter of the inner periphery small diameter portion 19, at least at the bottom end part at the part which enters inside the inner periphery small diameter portion 19. Accordingly, in a state with the first intermediate material 11 set inside the die 13, the bottom surface of the first intermediate material 11 contacts the inner portion of the annular groove 17 at the top surface of the base plate 15, as shown by (C). Then from this state, the punch 14 is lowered by the ram, thereby compressing the center portion of the first intermediate material 11 in the axial direction between the forward end face of the punch 14 and the top surface of the base plate 15 of the die 13, as shown by (D).
The metal material which is extruded radially outward from the space between the top surface of the base plate 15 and the forward end face of the punch 14 by this compressing action, moves in the opposite direction (upward) to the pressing direction of the punch 14, together with the metal material present at the radially outlying portions of the first intermediate material 11. In this manner, the metal material which moves in the opposite direction to the pressing direction of the punch 14 follows the shape of the outer peripheral surface of the punch 14 and the inner peripheral surface of the peripheral wall portion 16 to form a stepped cylinder whose inner and outer peripheral surfaces are stepped cylindrical surfaces. Furthermore, some of the metal material enters inside the annular groove 17, so that the shape of this portion becomes a rim shape. By the backward extrusion process performed in this manner, a second intermediate material 12 is obtained in the overall shape of a bottomed cylinder in which the inner and outer peripheral surfaces are stepped cylindrical surfaces as shown by (D).
Next, this second intermediate material 12 is subjected to a punching process which punches out a base portion 24 of the second intermediate material 12, to produce a third intermediate material 25 in the shape of a stepped cylinder as shown by (E). This punching process is performed by using a pressing machine to drive a blanking punch through the second intermediate material 12.
After the third intermediate material 25 is produced in this manner, the third intermediate material 25 is subjected to a cold roll forming (CRF) to produce a fourth intermediate material 26 shown by (F). In this cold roll forming, for example the third intermediate material 25 is fitted inside an external diameter side roller which has an inside diameter matching the outside diameter (on the large diameter side) of the third intermediate material 25 and whose inner peripheral surface is a cylindrical surface. Moreover an internal diameter side roller which has an outside diameter sufficiently smaller than the inside diameter of the third intermediate material 25 and whose outer peripheral surface generating line shape corresponds with the generating line shape of the inner peripheral surface of the fourth intermediate material 26 (in opposite relief) is pushed against the inner peripheral surface of the third intermediate material 25. Then, while rotating the internal diameter side roller, it is pushed against the inner peripheral surface of the third intermediate material 25. Because the external diameter side roller is supported in a manner which allows only rotation (in a state where displacement in the radial direction is prevented), then with rotation of the internal diameter side roller, the third intermediate material 25 rotates together with the external diameter side roller. As a result, the generating line shape of the outer peripheral surface of the internal diameter side roller is transferred to the entire periphery of the inner peripheral surface of the third intermediate material 25, and the outer peripheral surface of the third intermediate material 25 is processed into a cylindrical surface.
This rolling process may also be performed by sandwiching part of the third intermediate material 25 between a pair of rollers rotating in mutually opposite directions, and applying pressure to the rollers to push them towards each other so as to transfer the shape of the outer peripheral surfaces of the rollers to the inner and outer peripheral surfaces of the third intermediate material 25. In either case, the fourth intermediate material 26 as shown by (F) is obtained. In this fourth intermediate material 26, the outer peripheral surface forms a cylindrical surface whose outside diameter does not vary substantially in relation to the axial direction, and the inner peripheral surface has an inclined shape where an inside diameter is smallest at the axial center portion and the inside diameter increases gradually towards both axial direction end portions.
The thus obtained fourth intermediate material 26 is subjected to the required finishing processes to thereby complete the outer ring 3 which constitutes the double-row angular ball bearing 1, as shown in FIG. 7. That is to say, by shaving away the excess portion of the fourth intermediate material 26, the outer ring 3 with the shape indicated by the chain lines in FIGS. 8 (F) and FIG. 9 is obtained. Furthermore, the part corresponding to the pair of outer ring raceways 2 formed in the inner peripheral surface of the outer ring 3, is subjected to processes such as a grinding and superfinishing to enhance the surface characteristics of the two outer ring raceways 2.
Incidentally, for the raw material 10 for making the outer ring 3, a column shaped material is used which is made by cutting to predetermined lengths, a long piece of material with a circular cross-section that has been extrusion-molded by a steelmaker. The fact that the composition (cleanliness) of the column shaped raw material 10 obtained in this manner is not uniform, that is the range of the central 40% of the raw material 10 (the central cylindrical section from the core to 40% of the radius) tends to contain non-metallic inclusions, is already well known from descriptions in patent document 6 and other sources. Also known is that in relation to the range of the diametrically outermost 20% of the raw material 10 (the cylindrical section existing on the outer peripheral side more than 80% of the radius from the center), the cleanliness is low due to the susceptibility to the presence of oxides and non-metallic inclusions. Moreover, when metal material with low cleanliness, whether from the center or near the outer peripheral surface, is exposed at the outer ring raceways 2 provided on the inner peripheral surface of the outer ring 3, particularly the part that makes rolling contact with the rolling surface of the ball 6 (FIG. 7), ensuring the rolling fatigue life of these parts is difficult.
When these circumstances are considered, and also variations in the distribution of oxides and non-metallic inclusions within the material as well as various differences that occur at the time of the manufacturing operation (such as compressive force) are considered, the metal material present in the range of the central 50% of the raw material 10 and in the range of the outermost 30% of the raw material 10 is preferably not exposed in the outer ring raceways 2, at least in those parts which make rolling contact with the rolling surface. In other words, at least in the parts of the outer ring raceways 3 which make rolling contact with the rolling surface, preferably the metal material present in a middle cylindrical portion 27 of the raw material 10 (the crosshatched parts of (A) in FIG. 8. The other crosshatched parts in FIGS. 1 to FIG. 6, FIGS. 8 (B) to (F), and FIG. 9 show that these are also composed of the metal material (middle metal material 29) present in the middle cylindrical portion 27), in a range from 50 to 70% of the radius from the center, is exposed.
Incidentally, when a forging process is used to manufacture an outer ring 3 of the type which is the object of the present invention, having a small inside diameter at the axial center portion and comprising double row outer ring raceways at two locations in the axial direction on the inner peripheral surface on either side of this small diameter portion, exposing the metal material present in the middle cylindrical portion 27 to the two raceway surfaces is difficult. For example, when the outer ring 3 shown by the chain line in FIG. 9 is produced by a method such as shown in FIG. 8 above, metal material of each part in the raw material 10, that is the core metal material 28 which is present in the central columnar portion from the center to 50% of the radius, the middle metal material 29 which is present in the middle cylindrical portion 27, that is a range of 50 to 70% of the radius from the center, and the outer metal material 30 which is present in the outlying cylindrical portion, that is a range of the outermost 30%, is distributed throughout the outer ring 3 as shown in FIG. 9. For this outer ring 3, as described above, the fourth intermediate material 26 as shown by the solid line in FIG. 9 is produced by a forging process, after which the fourth intermediate material 26 is shaved off to the state shown by the chain line in FIG. 9, by machining and grinding processes, and completed as the outer ring 3.
In FIG. 9 which shows the fourth intermediate material 26 and the outer ring 3, if the middle metal material 29 present in the middle cylindrical portion 27 shown by the crosshatching is exposed at least at the part of the pair of outer ring raceways 2 which makes rolling contact with the rolling surface of the ball, the rolling fatigue life of these two outer ring raceways 2 is ensured, which easily ensures the durability of the double-row angular ball bearing 1 which includes this outer ring 3. However, as is clear from FIG. 9, when the outer ring 3 is made by the conventional manufacturing method, the core metal material 28 of the central columnar portion is exposed on the entire surface of one outer ring raceway 2 of the two outer ring raceways 2 (the lower outer ring raceway 2 in FIG. 9). For example, the arrows a in FIG. 9 indicate the direction of action of the load applied from the balls 6 to the two outer ring raceways 2 (see FIG. 7) in the case where the contact angle of the balls 6 is 40° (the complementary angle of the contact angle relative to the center axis is 50°. If the middle metal material 29 is present at the part indicated by the arrows α on the chain line in FIG. 9 which indicates the cross-sectional shape of the outer ring 3, then the rolling fatigue life of the two outer ring raceways 2 can be ensured easily. However in relation to the lower inner ring raceway 2 of FIG. 9, the core metal material 28 is present in the part on the chain line indicated by the arrows α. Consequently, in conventionally known methods of manufacturing bearing outer rings, the degree of freedom in the design for ensuring the durability of the double-row angular ball bearing 1 is limited.    Patent document 1: Japanese Patent Application Publication No. Hei 9-176740    Patent document 2: Japanese Patent Application Publication No. Hei 9-280255    Patent document 3: Japanese Patent Application Publication No. Hei 11-140543    Patent document 4: Japanese Patent Application Publication No. 2002-79347    Patent document 5: Japanese Patent Application Publication No. 2003-230927    Patent document 6: Japanese Patent Application Publication No. 2006-250317